I'm not too familiar with https://cube.dev/ - but my initial impression is they are focused more on providing APIs backed by SQL. They have a SQL API that emulates the PostgreSQL wire protocol, whereas Spice implements Arrow and Flight SQL natively. Their pre-aggregations are a similar concept to Spice's data accelerators. It also looks like they have their own query language, whereas Spice is native SQL as well.
DuckDB is an in-process DB similar to SQLite - so every application in your stack would need to embed it. Spice is a binary that has Flight SQL and HTTP query endpoints - so multiple applications can connect to it from any language.
